A PORTRAIT of Robert Louis Stevenson, the poet and author of Treasure Island has sold for (pounds) 4.9m in the United States.

Sotheby's, the auctioneers, said in New York that the work by John Singer Sargent, an American painter, had been bought by Steve Wynn, a casino owner.

The businessman who a year ago bought a Cezanne and a Renoir for a total of (pounds) 22.9m said the painting would be included in the art collection at his Las Vegas Resort and Country Club next year.

Sargent's iconic portrait of Stevenson and his wife Fanny, which one US art expert said was ''one of the artist's most daring and modern portraits'' was painted in 1885 when he visited Bournemouth. Stevenson had described the portrait as too eccentric to be exhibited.

It was a gift from the artist to the Stevensons, whose delight in it was described by Fanny in a letter to her mother-in-law as ''like an open box of jewels''.

Far from the formal portraits of the day, the work shows the author striding across a room in a velveteen jacket, twirling his moustache, while his wife, dressed in Indian costume, sits behind and to the side.

The portrait and two other Sargent paintings in Sotheby's sale of American art sold for a total of (pounds) 9.6m.

All three were once in the private collection of US newspaper owner and diplomat John Hay ''Jock'' Whitney.

The Greentree Foundation will use the money from the art sales to further its work of promoting international co-operation, peace and human rights.

Sargent's Venetian Loggia, a scene of women in an arcade, sold for (pounds) 3.1m.

A watercolour, Madame Roger-Jourdain, showing Sargent's Parisian friend and neighbour with parasol, lying on grass, went for (pounds) 1.5m.

All three works attracted prices above Sotheby's highest estimates before the sale.